We have shown that a purified growth factor, fibroblast growth factor (FGF), stimulates early changes in phosphorylation in 3T3 cells. The phosphorylated components, resolved by SDS polyacrylamide gel electrophoresis, are degraded by both trypsin and pronase. A change in phosphorylation of a 33,000 plus or minus 4,000 dalton component has been observed as early as five minutes after addition of FGF. This component is localized in the microsomal fraction of the cell. We have preliminary evidence that we can observe stimulation of phosphorylation by FGF in an in vivo assay. We propose to determine whether early changes in phosphorylation due to FGF could be causal for later increases in DNA synthesis. Our approach will be to 1) determine the earliest change in phosphoprotein phosphorylation which occurs after the addition of FGF to quiescent 3T3 cells; 2) correlate the effect of FGF on DNA synthesis with its effect on the earliest phosphorylation change; 3) characterize the product(s) of this phosphorylation reaction; 4) characterize the earliest phosphorylation reaction induced by FGF; and 5) establish an in vitro system to study FGF stimulation of phosphorylation. These studies will involve a comparison of control of phosphorylation in normal and transformed cells. Thus, they will provide information as to whether these controls are altered in the transformed cell.